1. Field of the Invention
This invention relates to a production process of an SOI (Si on Insulator) substrate, which has a promising prospect as a substrate material for next-generation LSIs and carries a semiconductor layer on an insulator.
2. Description of the Prior Art
As a process for forming on an insulator an SOI structure having a semiconductor active layer therein, there is the SIMOX (Separation by Implanted Oxygen) process as disclosed, for example, in J. Mater. Res., 8(3), 523-534 (1993), in which O.sup.+ ions are implanted at a high dose in an Si substrate, followed by heat treatment at elevated temperature so that a continuous oxide film (SiO.sub.2 film) is formed inside the substrate.
This process, while permitting rather easy provision of an SOI substrate, is accompanied by a drawback in that crystal defects remain in an upper Si active layer which is a device-forming region. The density of residual crystal defects depends upon the dose of implanted oxygen ions, so that a high dose leads to many residual crystal defects compared with a low dose. For a reduction in crystal defects, it is therefore necessary to control the dose of implanted oxygen ions low. From the viewpoint of achieving a reduction in production cost, a low dose is also desired as the time for ion implantation can be shortened. However, when oxygen ions are implanted at a low dose of 3.5.times.10.sup.17 ions/cm.sup.2 or lower and 180 KeV, for example, the subsequent high-temperature heat treatment cannot form any continuous oxide film but results in the formation of paths for current leakage, thereby failing to obtain good device characteristics.
Namely, the SIMOX process has the demerit that crystal defects remain in an upper Si active layer which is a region where devices are formed, although it has the merit that an SOI substrate can be obtained rather easily. Further, the implantation of oxygen ions at a low dose with a view to reducing crystal defects and production cost is accompanied by the drawback that the subsequent high-temperature heat treatment cannot form any continuous oxide film but results in the formation of paths for current leakage, thereby failing to obtain good device characteristics.
An object of the present invention is to overcome the above-described problems of SIMOX and to provide a production process for obtaining at low cost an SIMOX substrate with reduced crystal defects.